Vehicle service system optical target assembly calibration

ABSTRACT

A machine vision vehicle wheel alignment system for acquiring measurements associated with a vehicle. The system includes at least one imaging sensor having a field of view and at least one optical target secured to a wheel assembly on a vehicle within the field of view of the imaging sensor. The optical target includes a plurality of visible target elements disposed on at least two surfaces in a determinable geometric and spatial configuration which are calibrated prior to use. A processing unit in the system is configured to receive at least two sets of image data from the imaging sensor, with each set of image data acquired at a different rotational position of the wheel assembly around an axis of rotation and representative of at least one visible target element on each of the two surfaces, from which the processing unit is configured to identify said axis of rotation of the wheel assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,co-pending U.S. patent application Ser. No. 13/781,977 filed on Mar. 1,2013, which in turn is a continuation of U.S. patent application Ser.No. 13/483,976 filed on May 30, 2012, now U.S. Pat. No. 8,490,290 B2.The '290 patent is a continuation of U.S. patent application Ser. No.13/226,829 filed on Sep. 7, 2011, now U.S. Pat. No. 8,215,023 B2. The'023 patent is a continuation of, and claims priority to, U.S. patentapplication Ser. No. 13/071,172 filed on Mar. 24, 2011, now U.S. Pat.No. 8,033,028, B2, which in turn is a continuation of, and claimspriority to, U.S. patent application Ser. No. 12/720,453 filed on Mar.9, 2010, now U.S. Pat. No. 7,930,834 B2. The '834 patent is acontinuation of U.S. patent application Ser. No. 12/172,554 filed onJul. 14, 2008, now U.S. Pat. No. 7,703,212, which in turn is acontinuation of U.S. patent application Ser. No. 11/535,881 filed onSep. 27, 2006, now U.S. Pat. No. 7,444,752, which in turn is related to,and claims priority from, U.S. Provisional Patent Application Ser. No.60/721,206 filed on Sep. 28, 2005. Each of the aforementionedapplications and patents are herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to machine vision vehicle service systems,and in particular to an optical target assembly configured for mountingto a surface of a vehicle, such as a vehicle wheel, during amachine-vision vehicle wheel alignment procedure.

A machine-vision vehicle service system, such as a vehicle wheelalignment system like the Series 811 Wheel Alignment System utilizingthe DSP 600 Series sensors, manufactured and sold by Hunter EngineeringCompany of Bridgeton, Mo., consists generally of a console having acomputer or processing unit, one or more display devices such as amonitor, and one or more input devices such as a keyboard. In amachine-vision vehicle wheel alignment system, one or more imagingsensor arrays are mounted away from a vehicle undergoing an alignmentinspection, and are configured to obtain images of alignment targets orother identifiable features associated with the vehicle forcommunication to the processing unit. Correspondingly, the processingunit is configured with one or more software applications, at least oneof which is adapted to facilitate the alignment of vehicle wheels whichgenerally consist of a rim and an associated tire, using input receivedfrom the imaging sensors.

The machine-vision imaging sensors are traditionally part of a camerasystem or imaging system configured to view optical targets withinassociated fields of view to obtain images thereof for processing by thesoftware applications in the console. Commonly, the observed opticaltargets incorporate highly accurate patterns that have known controlfeatures. The positions and relationships of the features in the imagesare determined, and the orientation of the wheels or other vehiclecomponents to which the optical targets are attached are calculated bywell known algorithms. Exemplary configurations for the high-accuracyoptical targets are described in U.S. Pat. No. 6,064,750 to January, andin U.S. Pat. No. 6,134,792 to January. Each optical target consists of atarget face, on which are disposed identifiable optical elements, aprecision flat base, and a mounting shaft adapted for attachment to aseparate clamping assembly secured to the vehicle or vehicle wheelassembly.

The conventional configuration for an optical target is preciselyengineered with high-contrast optical elements such as circles, squares,or triangles. The accuracy of such conventionally configured opticaltargets is dependant upon how well the high contrast edges of theoptical target elements can be located in an image produced by theimaging components of the wheel alignment system. For the best accuracy,the individual optical elements must be large enough to have relativelylong straight or curved boundaries, and they must be separated farenough to prevent the individual optical target elements from appearingto fuse into a single object when reduced edge sharpness causes two ormore optical target elements to bleed into the same pixel in the imagingsystem. These factors combine to limit the number of individual imagepixels generated by the imaging system whose values are utilized tocalculate a position and orientation of a conventionally configuredoptical target.

Each image of conventional high-contrast optical target acquired by theoptical imaging vehicle wheel alignment system is processed to identifya number of reference points in the image. Either the computer or theimaging system is configured to mathematically manipulate the positionalrelationships of the observed reference points, as identified in animage, to match them with a set of predetermined positionalrelationships based on the known parameters of the conventionalhigh-contrast optical target. Once the relationship between the observedpositional relationships and the predetermined positional relationshipsis identified for the reference points, the position and orientation inthree-dimensional space of the target (and an associated vehicle wheel)relative to the position and orientation of the imaging system isidentified, from which one or more vehicle wheel alignment angles can beidentified. Accordingly, for an optical imaging vehicle wheel alignmentsystem to function, it is necessary for the system to be capable ofextracting a set of control or reference points from acquired images.

To further facilitate the operation of a machine vision vehicle wheelalignment system, the separate optical targets are secured to thevehicle wheels with precision wheel adaptors configured to clamp ontothe vehicle wheel edges and to position a mounting point for the opticaltarget substantially coaxial with the wheel rim's axis of rotation. Thetraditional precision wheel adaptors typically include a set of claws orfeet adapted to secure the wheel adaptor to the vehicle wheel assemblyby engaging the lip or rim of the wheel rim at the tire junction. Acentering mechanism on the wheel adaptor ensures that the claws or feetof the wheel adaptor are adjusted in a symmetrical manner to maintainthe mounting point for the optical target in a determined centeredconfiguration in relation to the axial center of the wheel rim.

Some variations of traditional wheel adaptors, such as the Tire ClampAdaptor Model No. 20-1789-1 from Hunter Engineering Co., and those shownin U.S. Pat. No. 5,987,761 to Ohnesorge and U.S. Pat. No. 6,131,293 toMaioli et al. further utilize a set of gripping arms adapted to engagetire surfaces in conjunction with a set of contact supports andcentering mechanisms for symmetrically engaging the circumferential lipof the wheel rim and securing the wheel adaptors in an axially centeredposition on the vehicle wheel assembly.

Other vehicle-specific wheel adaptors, such as those for use withMercedes Benz and BMW automobiles, are configured with a set of pinswhich are designed to pass through the wheel assembly, and to contactpredetermined surfaces on the vehicle wheel hubs, positioning thevehicle-specific wheel adaptor in a predetermined axially centeredlocation about the wheel assembly. These vehicle specific wheel adaptorsare then held in place by means of tire clamps or spring mechanismswhich grip to the tire tread surfaces.

Traditional wheel adaptors that will universally adapt to the wide rangeof wheel sizes on the market today are difficult to design and costly tobuild. Many times additional parts are required, such as extenders, inorder to allow the adaptor to work with wheels that are very small orvery large which also adds additional cost and complication to theadaptor system. Additionally, traditional adaptors have to provide asubstantial amount of clamping force in order to hold the weight of thetarget or sensor on the wheel assembly. This clamping force can scratchor dent the wheel assembly where it is attached. This is veryundesirable especially when the wheel assembly is a very costlyaftermarket wheel.

Accordingly, it would be advantageous to provide a machine visionvehicle service system, such as a wheel alignment system, with anoptical target assembly which incorporates both the optical target and asimplified adaptor for attachment to a vehicle wheel, and which does notrequire a determined precision mounting on the vehicle wheel assembly inrelation to the wheel axis of rotation.

It would be further advantageous to provide a machine vision vehiclewheel alignment system with a mechanically simplified optical targetassembly which is light weight, dimensionally stable, less abrasive tothe wheel rim surfaces, and which does not require precisionconstruction.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, the present invention provides a machine vision vehiclewheel alignment system optical target assembly which incorporates anadaptor for attachment of an optical target to a vehicle wheel assembly.The adaptor includes at least one fixed contact point for seating in anon-determined position against surfaces of a vehicle wheel assembly,and a pair of clamping arms configured to grip tread surfaces of a tiremounted to the wheel rim to hold the optical target assembly in contactwith the wheel assembly surface. The optical target is secured to theadaptor, and maintained in a stationary relationship to the wheelassembly thereby during a vehicle wheel alignment procedure.

In an alternate embodiment, the present invention provides an improvedmachine vision vehicle wheel alignment system having a processing unitand at least one imaging sensor for acquiring images of a non-planartarget having a plurality of visible features. The non-planar target isremovably secured against a vehicle wheel assembly surface, within afield of view of the imaging sensor. A processing unit receives datarepresentative of said visible features from the imaging sensor, anddetermines at least an axis of rotation of the vehicle wheel utilizingthe received data.

The foregoing and other objects, features, and advantages of theinvention as well as presently preferred embodiments thereof will becomemore apparent from the reading of the following description inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the accompanying drawings which form part of the specification:

FIG. 1 is a perspective rear view of a first embodiment of the opticaltarget assembly of the present invention;

FIG. 2 is perspective rear view of the embodiment of FIG. 1 secured to avehicle wheel assembly;

FIG. 3 is a perspective front view of FIG. 2;

FIG. 4 is a perspective side view of an alternate embodiment of theoptical target assembly of the present invention;

FIG. 5 is a perspective front view of the embodiment shown in FIG. 4;

FIG. 6 is a perspective rear view of the embodiment shown in FIG. 4;

FIG. 7 is a perspective front view of a second alternate embodiment ofthe optical target assembly of the present invention;

FIG. 8 is a perspective illustration of an optical target assembly ofFIG. 1 mounted to a vehicle wheel, rotated between two positions aboutan axis of rotation; and

FIG. 9 is a simplified representation of reference points and associatedinterrelationships on an optical target face.

Corresponding reference numerals indicate corresponding parts throughoutthe several figures of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description illustrates the invention by way ofexample and not by way of limitation. The description clearly enablesone skilled in the art to make and use the invention, describes severalembodiments, adaptations, variations, alternatives, and uses of theinvention, including what is presently believed to be the best mode ofcarrying out the invention.

Turning to FIG. 1, an embodiment of the optical target assembly 100 ofthe present invention is shown in a rear perspective view. The opticaltarget assembly 100 consists of a simplified or unitary base assembly102, a pair of opposed wheel clamp arms 108A and 108B coupled to thebase assembly, a target support assembly 104 coupled to the baseassembly 102, and an optical target 106 secured to the target supportassembly.

The simplified or unitary base assembly 102 defines one or more fixedcontact surfaces 110 adapted for abutting contact with the generallyvertical outer surfaces of a vehicle wheel rim 10 between the outercircumferential lip of the wheel rim and the wheel assembly axis ofrotation. The fixed contact surfaces 110 are disposed in a common plane,and as shown in FIG. 1, and are disposed equidistantly about thecircumference of an annular member 112, defining a central opening 114adapted to surround an axial hub-end portion of the vehicle wheel rim10. A raised support assembly 116 is coaxially secured to the annularmember 112 opposite from the fixed contact surfaces 110, extending overthe central opening 114 to provide clearance for the hub-end portion ofthe vehicle wheel rim 10 and to provide support for the various attachedcomponents. Since the assembly 102 of the present invention is intendedfor a non-determined placement against the wheel assembly surface, theassembly 102 does not include any adjustment mechanisms for centeringthe contact surfaces 110 relative to either the wheel rimcircumferential lip or axis of rotation, such as those commonly found inself-centering or adjustable wheel adaptors.

To hold the fixed contact surfaces 110 in engagement with a generallyvertical surface of a vehicle wheel rim 10, as shown in FIG. 2, a pairof opposed wheel clamp arms 108A and 108B coupled to the supportassembly 116 of the base assembly 102. Each wheel clamp arm 108 includesa tire hook 122 disposed on an end opposite from the support assembly116. The tire hook is configured to grip a tire tread surface of avehicle wheel 12. Each opposed wheel clamp arm 108 is adjustable toaccommodate wheel assemblies of different dimensions.

For example, as shown in FIGS. 1 and 2, the wheel clamp arms 108A and108B may each be extended or retracted along a longitudinal axis,pivoted about an attachment point to the support assembly 116, oradjusted vertically relative to the attachment point. Those of ordinaryskill in the art will recognize that any of a variety of mechanicalcomponents may be utilized to achieve the desired range of movement forthe wheel clamp arms 108, including, but not limited to, slidingassemblies, threaded assemblies, pivoting assemblies, and expandingassemblies. It is not required that the wheel clamp arms 108 be adjustedsynchronously, or that they be disposed in mirror-image configurations,provided the wheel clamp arms 108 are sufficiently positioned to engagethe tire tread surfaces of a vehicle wheel to secure the fixed contactsurfaces 110 of the base assembly 102 against the surfaces of the wheelrim 10 between the circumferential lip and the axial center point, in astationary and stable manner during wheel alignment angle measurementsand procedures.

As seen in FIGS. 1-3, the target support assembly 104 is coaxiallysecured to the support assembly 116, and comprises a coaxial targetsupport shaft 118 and a target support flange 120 secured to the targetsupport shaft 118. The target support flange 120 is adjustably coupledto the end of the target support shaft 118, and is configured to supportthe optical target 106 at an angled orientation relative to thelongitudinal axis of the target support shaft 118.

The optical target 106 provides visible features which are identifiablein images acquired by an imaging system associated with a vehicleservice device, and which provide a sufficient number of data points toenable a determination as to the position and orientation of the opticaltarget 106 in three-dimensional space from acquired images. For example,the optical target 106 may include a set of geometric figures arrangedin a predetermined configuration as shown in U.S. Pat. No. 6,134,792 toJanuary, herein incorporated by reference, or simply a set ofidentifiable fixed features, such as shown in U.S. Pat. No. 6,894,771 toDorrance et al., herein incorporated by reference. The visible features(data points) of the optical target 106 need not be disposed on a planarsurface, but rather, may be disposed on any dimensionally stable surfaceor shape, including non-planar surfaces, smoothly curved surfaces, orangled surfaces.

It is less costly to manufacture a target where the identifiable fixedfeatures are not precisely known. U.S. Pat. No. 6,894,771 to Dorrance etal. describes an optimization method that may be employed to preciselydetermine the location of the features by acquiring multiple images ofthe target while it is rotated. This method could be used on everyalignment that is performed when the vehicle is rolled on the runwaysurface to determine the axis of rotation, or the method may beperformed once as a target calibration procedure where the location ofthe features are stored and later used via methods outlined by U.S. Pat.No. 6,134,792 to January.

In an alternate embodiment of the present invention, shown in FIGS. 4-6,an offset target support flange 150 is adjustably coupled to the end ofthe target support shaft 118, and is configured to support the opticaltarget 106 at an angled orientation offset relative to the longitudinalaxis of the target support shaft 118. Specifically, as is best seen inFIG. 5, the optical target 106 is secured to the offset target supportflange 150 and maintained in an outward position from the vehicle wheelassembly which is below, and in-front of, the axis of rotation AR of thevehicle wheel assembly when mounted thereon.

By positioning the optical target 106 in an offset position which isbelow and in-front of the wheel assembly axis of rotation AR, theoptical target 106 remains visible within the field of view of imagingsensors disposed in-front of the vehicle over a wider range of steeringmovement for the vehicle wheel assembly. As a vehicle wheel assembly issteered in an outward direction about a steering axis, the leading edgeof the tire 12 will occlude a line of sight between an imaging sensordisposed in-front of the vehicle and an optical target 106 disposedsubstantially coaxial with the vehicle wheel assembly axis of rotationAR. To counter-act this effect, coaxially mounted optical targets 106generally must be disposed sufficiently outward from the surface of thevehicle wheel assembly so as to remain visible throughout the desiredrange of steering angles. As the optical target mounting is movedoutward from the surface of the vehicle wheel rim 10, the weight of theoptical target 106 acts on the various target support assemblies 100over a greater distance, requiring a corresponding increase inattachment strength to maintain the optical target 106 in a securedposition against the vehicle wheel assembly. Accordingly, by disposingthe optical target 106 below and in-front of the vehicle wheel axis ofrotation AR, the leading edge of the tire 12 does not occlude as much ofthe optical target surface during a steering movement, andcorresponding, the optical target 106 does not have to be disposed asfar outward from the vehicle wheel rim surface 10 as an on-axis opticaltarget would be, resulting in a reduction in the attachment strengthrequired to maintain the offset optical target 106 in a secure positionagainst the vehicle wheel assembly surface.

Those of ordinary skill in the art will recognize that the specificpositioning of the optical target 106 off of the vehicle wheel assemblyaxis of rotation AR may vary, depending upon the particularconfiguration of the vehicle wheel assembly, adjacent vehicle bodycomponents, and the placement of the imaging sensors disposed to viewthe optical target 106. Accordingly, it is intended that any of avariety of optical target support assemblies, having differentconfigurations and attachment components for maintaining an opticaltarget 106 in a secure position and orientation relative to a vehiclewheel assembly surface may be utilized within the scope of theinvention.

For example, turning to FIG. 7, an alternate embodiment of the opticaltarget support assembly is shown generally at 200. The optical targetsupport assembly 200 is of unitary construction, incorporating anoptical target 206 directly onto an exterior surface of the supportassembly 200, opposite from one or more fixed contact surfaces 210. Asshown in FIG. 7, the optical target support assembly 200 is generallycoaxial with the vehicle wheel axis or rotation AR, but those ofordinary skill in the art will recognize that the optical target supportassembly 200 may be varied in shape, size, and configuration to positionthe incorporated optical target 206 into a surface of the supportassembly 200 at other desired positions and orientations relative to avehicle wheel axis of rotation, such as previously described. A base 202of the support assembly defines the fixed contact surfaces 210 adaptedto contact a generally vertical outer surface of a vehicle wheel rim 10between the circumferential lip of the wheel rim and the axis ofrotation AR. The contact surfaces 210 are disposed in a common plane,and as shown in FIG. 4, are disposed equidistantly about thecircumference of the base 202, adapted to surround an axial hub-endportion of the vehicle wheel rim 10. Preferably, the body is formed froma molded plastic or other lightweight material.

Any of a variety of mechanisms may be utilized to hold the opticaltarget support assembly 200 against the surface of a vehicle wheelassembly during use, provided the target support assembly 200 ismaintained in a fixed position against the surface of the vehicle wheelassembly. For example, wheel clamp arms (not shown) may be utilized insubstantially the same manner as previously described to grip the treadsurface of the vehicle wheel tire 12, or flexible “bungee” style elasticcords may be utilized to “pull” the optical target support assemblyagainst the face surfaces of the vehicle wheel rim 10. Similarly,removable adhesives or magnets of sufficient strength to support theoptical target support assembly 200 may be utilized on suitablesurfaces.

During use, an optical target support assembly of the present invention,such as shown at 100 or 200, is positioned against a generally verticalouter surface of a vehicle wheel rim 10, such that the visible featuresof the optical target 106, 206 associated with the support assembly areorientated for viewing by one or more imaging sensors. The specificplacement of the support assembly against the outer surface of the wheelrim 10 need not be coaxial with the wheel assembly, i.e., may beeccentric with the vehicle wheel assembly, but must be sufficientlystable to prevent the optical target from tilting, wobbling, or slippingfrom the initial position during a vehicle service procedure. Tomaintain the optical target support assembly in a stable positionagainst the wheel rim surface, suitable support mechanisms, such as thewheel clamp arms shown in FIGS. 2 and 3, are adjusted to grip treadsurfaces of the vehicle tire 12, preferably above a horizontal planethrough which the wheel axis of rotation AR passes. The interaction ofgravity, the optical target support mechanisms, and the fixed contactsurfaces of the optical target support assembly interact to maintain thefixed contact surfaces against the wheel rim surface, and to hold theoptical target 106, 206 in a stable position during a vehicle serviceprocedure and through a limited range of wheel assembly movement.

Since the optical target 106, 206 disposed on the optical target supportassembly is not secured to the vehicle wheel assembly in anypredetermined position, e.g., concentric with the wheel axis of rotationAR, it is necessary to determine the wheel axis of rotation AR forpurposes of calculating vehicle wheel alignment angle measurements fromimages of the optical targets 106, 206. It is also desirable to computethe wheel assembly center point to further increase accuracy of thealignment measurements. Methods to determine the axis of rotation andcenter point are described in U.S. Pat. No. 7,702,126 B2 to Strege etal, herein incorporated by reference.

For example, a three-dimensional direction vector corresponding to awheel assembly's axis of rotation AR and a point corresponding to thewheel assembly center point CP can be estimated from two or more opticaltarget location “snapshots”, taken as the wheel assembly is rotatedpartially about the axis of rotation AR. Mathematical techniques may beused to obtain a direction vector from the optical target location andattitude information contained in two or more snapshots. The acquiredsequence of images is utilized to identify an actual intersection pointIP between the axis of rotation AR of the vehicle wheel assembly and thesurface of the optical target 106, 206 as a point on the optical targetsurface having the least amount of linear deviation in the sequence ofimages.

A reference point, such as the wheel center point CP may be establishedfor a vehicle wheel using an optical target 106, 206 operatively coupledto the vehicle wheel in a predetermined relationship, such as shown inFIG. 8. Two or more images of the wheel-mounted optical target areacquired at different rotational positions of the vehicle wheel, and aposition and orientation of the wheel-mounted optical target isdetermined in a three-dimensional coordinated system from the acquiredimages.

The three-dimensional direction vector corresponding to a wheel's axisof rotation AR can also be estimated from two or more optical targetlocation “snapshots”, taken as the wheel assembly is rotated about theaxis of rotation AR. Mathematical techniques may be used to obtain adirection vector from the target location and attitude informationcontained in two or more snapshots. Although direction information canbe obtained, specific points in space which are known to lie on thewheel's axis-of-rotation AR are not identified.

Using the predetermined relationship between the wheel-mounted opticaltarget with the vehicle wheel, and the predetermined configuration ofthe optical target, an intersection point IP is identified in thethree-dimensional coordinate system between an axis of rotation AR ofthe vehicle wheel and an observed surface of the optical target. Thisintersection point IP is referred to as the “nominal piercing point”.Assuming that the actual configuration of the wheel and the opticaltarget conforms with the predetermined configurations thereof, thepredetermined configuration of the wheel-mounted optical target and theintersection point IP are utilized to identify a point on the axis ofrotation AR which is displaced from the optical target surface by apredetermined distance, based on the known configuration of the opticaltarget and vehicle wheel. The identified point corresponds to the wheelcenter point CP of the vehicle wheel.

Alternatively, a wheel center point CP for a vehicle wheel may bedetermined from a sequence of images of an optical target operativelycoupled to the vehicle wheel. The vehicle wheel is rotated about theaxis of rotation AR while a sequence of at least two images of thewheel-mounted optical target is acquired. The acquired sequence ofimages is utilized by the vehicle wheel alignment system to identify anactual intersection point IP between the axis of rotation AR of thevehicle wheel and the face of the optical target as a point on theoptical target surface having the least amount of linear deviation inthe sequence of images.

Those of ordinary skill in the art will recognize that several methodsmay be utilized to identify the actual intersection point IP on thetarget surface, for example, a non-linear optimization technique such asthe Levenberg-Marquardt algorithm may be employed to solve anover-determined system of simultaneous equations to identify the pointon the optical target surface having the least amount of linear movementbetween two or more sequential images. However, non-linear optimizationtechniques are preferably utilized in situations where the wheelassembly is permitted to roll in a linear direction and a series ofimages of the optical target surface are captured. It is assumed thatthe axis of rotation AR of the wheel assembly will continue to point inthe same direction in each of the images.

Alternatively, planes TP1 and TP2 which are defined by the features onthe optical target surface are identified in each image of the opticaltarget 106, 206 at two different rotational positions of the vehiclewheel assembly, without requiring translational movement of the vehiclewheel assembly, i.e. when a vehicle is jacked up above a supportingsurface and the wheel assembly is not resting on the surface duringrotation. An equation representative of a line of intersection betweeneach of the two planes TP1 and TP2 is determined in a coordinate systembased on the first plane TP1, and next in a coordinate system based onthe second plane TP2. There are therefore two separate planar lines X1and X2 determined in the two separate planar coordinate systems and onlyone line AR determined by the intersection of the two planes TP1 and TP2in a three dimensional coordinate system. The single point ofintersection of the two lines X1 and X2 in a common coordinate system,preferably the coordinate system of the optical target surface,represents the actual intersection point IP between the axis of rotationAR of the wheel assembly and the optical target surface.

The actual intersection point IP, together with the configuration of theoptical target are utilized to identify a point on the axis of rotationAR displaced from the target surface, corresponding to the wheel centerpoint CP for the vehicle wheel assembly. With reference to FIG. 6, themethod for determining the actual intersection point IP usingintersecting lines is described in more detail. An optical targetsurface coordinate system is initially established based on visiblefeatures F of the optical target 106, 206. Precise coordinates arepredetermined for all the features F on the optical target surface. Theorigin of the optical target surface coordinates is based on a feature(real or determined) at the center of the array of visible features F.Four feature points F1, F5, F7, and F11 around the periphery of theoptical target surface are used in the method. These four outer featuresF of the optical target surface are selected to each lie at the sameradius from the target origin O. Although many sets of four features Fcould be employed, the preferred method uses features arrayed around theoptical target surface in a manner analogous to particular hours on aclock face. Feature F1 corresponds to one o'clock, feature F5corresponds to five o'clock, feature F7 corresponds to seven o'clock andfeature F11 corresponds to eleven o'clock.

The optical target is generally positioned relative to the vehicle wheelassembly by the optical target support assembly 102, 202 such that theaxis-of-rotation AR is not normal and not parallel to the optical targetsurface. There are four coordinate systems involved in the method, animaging system or camera coordinate system that identifies the positionand attitude of objects relative to the imaging system or camerameasurement sensors of a vehicle service system, an optical targetsurface coordinate system that identifies the location of optical targetfeatures relative to the center of the pattern on the optical targetsurface, and are two snapshot coordinate systems, Snap1 and Snap2.

The Snap1 coordinate system is such, that any point in space is assignedcoordinates that match the optical target surface coordinates the pointwould have when the optical target surface is aligned with the firstimage snapshot position. Analogously, the Snap2 coordinate systemmatches optical target surface coordinates when the optical targetsurface is aligned with the second image snapshot position.

An instantaneous coordinate transform between optical target coordinatesand camera coordinates is computed. Therefore, the two image snapshotsfacilitate computation of coordinate transforms between all four of theabove-mentioned coordinate systems.

Initially, the Snap1 coordinates of features F11 and F1 are obtained.These will be the same as the optical target surface coordinates ofthose features, according to the precise design model of the opticaltarget. The coordinates of these features are transformed into Snap2coordinates. Both points will probably be outside the Snap2 opticaltarget face plane (which would have a z coordinate of zero). A line isdefined between the two points and the point R where that lineintersects the Snap2 optical target face plane is identified. Point Rlies in both snapshot planes. The set of coordinates that describe pointR in Snap1 coordinates is designated R1, and the set of coordinates thatdescribe point R in Snap2 coordinates is designated R2. Because R2 liesin the second snapshot optical target face plane, the z-coordinate of R2is zero.

The known transform between the coordinate systems is applied to findthe coordinates of point R in Snap1 coordinates, i.e. R1. Because R1lies in the first snapshot optical target face plane, the z-coordinateof R1 is zero.

Next, the Snap1 coordinates of where features F7 and F5 appeared duringthe first snapshot are obtained. These will be the same as the opticaltarget coordinates of those features, according to the model of theoptical target. The two points are transformed into Snap2 coordinates.Both points may be outside the Snap2 optical target face plane (whichwould have a z coordinate of zero). A line is formed between the twopoints and the point where that line intersects the Snap2 optical targetface plane is identified. The intersection in space will be called pointS, and lies in both snapshot planes. Together, points R and S define theline in space where the two snapshot planes intersect. The set ofcoordinates that describe point S in Snap2 coordinates will be calledS2. Because S2 lies in the second snapshot target face plane, thez-coordinate of S2 is zero.

The known transform between coordinate systems is again applied to findthe coordinates of point S in Snap1 coordinates. These coordinates willbe referred to as S1. Because S1 lies in the first snapshot opticaltarget face plane, the z-coordinate of 51 is zero.

Next, R1, R2, 51 and S2 are identified on the same two-dimensional gridof target coordinates. All four have a z-coordinate of zero, so they maybe treated as X-Y coordinate pairs. The equation of the line joining thecoordinate pairs R1 and 51 is identified. This line represents the setof Snap1 coordinates for all points that lie on both snapshot planes.Next, the equation of the line that joins the coordinate pairs R2 and S2is identified, representing the set of Snap2 coordinates for all pointsthat lie on both snapshot planes. Solving these two line equationssimultaneously yields the X and Y optical target surface coordinates ofthe actual piercing point PP through which the axis of rotation of thevehicle wheel passes through the optical target surface.

The present invention can be embodied in part in the form ofcomputer-implemented processes and apparatuses for practicing thoseprocesses. The present invention can also be embodied in part in theform of computer program code containing instructions embodied intangible media, such as floppy diskettes, CD-ROMs, hard drives, oranother computer readable storage medium, wherein, when the computerprogram code is loaded into, and executed by, an electronic device suchas a computer, micro-processor or logic circuit, the device becomes anapparatus for practicing the invention.

The present invention can also be embodied in part in the form ofcomputer program code, for example, whether stored in a storage medium,loaded into and/or executed by a computer, or transmitted over sometransmission medium, such as over electrical wiring or cabling, throughfiber optics, or via electromagnetic radiation, wherein, when thecomputer program code is loaded into and executed by a computer, thecomputer becomes an apparatus for practicing the invention. Whenimplemented in a general-purpose microprocessor, the computer programcode segments configure the microprocessor to create specific logiccircuits.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results are obtained. Asvarious changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

The invention claimed is:
 1. A machine vision vehicle wheel alignmentsystem for acquiring measurements associated with a vehicle, comprising:at least one imaging sensor having a field of view; at least one opticaltarget secured to the vehicle within the field of view of said at leastone imaging sensor, said optical target having a plurality of visibletarget elements disposed on at least two surfaces in a determinablegeometric and spatial configuration; a processing unit configured toreceive image data representative of at least one visible target elementon each of said two surfaces from said at least one imaging sensor, andto process said received image data together with data establishing saiddeterminable geometric and spatial configuration of said plurality ofvisible target elements on said at least two surfaces, to determine atleast one measurement associated with said vehicle.
 2. The machinevision vehicle wheel alignment system of claim 1 wherein said at leasttwo surfaces are planar surfaces, are non-planar surfaces, are smoothlycurved surfaces, are angled surfaces, or are facets of a dimensionallystable object.
 3. The machine vision vehicle wheel alignment system ofclaim 1 wherein said at least two surfaces are dimensionally stable. 4.The machine vision vehicle wheel alignment system of claim 1 whereinsaid target elements on said first surface are arranged in apredetermined geometric and spatial configuration relative to eachother; wherein said target elements on said second surface are arrangedin a predetermined geometric and spatial configuration relative to eachother; wherein said processing unit is configured to determine at leastone measurement associated with said vehicle by utilizing saidpredetermined geometric and spatial relationships between said targetelements on said first surface, together with said predeterminedgeometric and spatial relationships between said target elements on saidsecond surface, to establish a geometric and spatial configuration ofsaid first surface relative to said second surface.
 5. The machinevision vehicle wheel alignment system of claim 1 wherein said processingunit is further configured to establish said determinable geometric andspatial configuration of said plurality of visible target elementsbefore determining said at least one measurement associated with saidvehicle.
 6. The machine vision vehicle wheel alignment system of claim 5wherein said processing unit is configured to establish saiddeterminable geometric and spatial configuration of said plurality ofvisible target elements from a plurality of images of said visibletarget elements, each of which is acquired at a different position orview of said optical target.
 7. The machine vision vehicle wheelalignment system of claim 1 wherein said data establishing saiddeterminable geometric and spatial configuration of said plurality ofvisible target elements further establishes a geometric and spatialconfiguration of said at least two surfaces on which said visible targetelements are disposed.
 8. The machine vision vehicle wheel alignmentsystem of claim 7 wherein said geometric and spatial configuration ofsaid at least two surfaces is defined as the geometric and spatialconfiguration of one of said surfaces relative to another of saidsurfaces.